A process of the latter type is described, for instance, in U.S. Pat. No. 4,175,233. In that process the deposition rate of solid constituents of the liquid to be examined (blood) are measured on germanium-coated glass plates. The glass plates, which are provided with electrode connections, are fitted into recesses in two shell halves and, together with two glass rods which are arranged on the longitudinal side between the glass plates, delimit, when the two shell halves are placed together, a cuboid space with open faces as the inflow and outflow of the flow cell. On the inflow and outflow sides there are provided two connections which are seated with flange-like widened parts in corresponding recesses in the shell halves. The connections each have at their opposite ends an annular groove which serves to fix a tube connection by means of a clamping ring. In a particular embodiment, one of the two shell halves has a light-permeable window in order to permit infra-red measurements as well. The two shell halves are held together in place by two clips. The flow cells thus formed are relatively large and accordingly insensitive to operate. Their range of use is mainly restricted to low-accuracy blood examinations.
Usually, however, flow cells are used which are designed first and foremost to carry out optical measuring processes. Such a flow cell is described in U.S. Pat. No. 4,243,883. This has a housing with a cover and, arranged inside the housing, a light source and a detector opposite it. The housing is provided with recesses to receive a coated flow tubing. In the measuring area between the light source and the detector, the cross-section of the tubing is narrowed and square in shape and equipped with windows in the coating which lie opposite each other. Said flow cell is used first and foremost for monitoring the blood of patients during operations, and the flow tubing must therefore have a fairly large cross-section. As a result of the integrated arrangement of the light source and the detector within the housing, the latter has relatively large dimensions and therefore can be operated relatively well. The possibilities for use of this flow cell are mainly restricted to the area stated.
Other, somewhat smaller flow cells are known from U.S. Pat. No. 3,728,032 and EP-A-No. 0,158,948. The two-part flow cell described in the US patent has an ellipsoid flow area with inlet and outlet openings lying opposite each other in the longitudinal direction of the cell. In the region of the measuring chamber where the diameter is greatest there are arranged two measuring windows lying opposite one another. The two halves of the flow cell, which are formed as mirror images, are connected by means of screws. The flow cell described in the European patent has convex depressions in the measuring area which are arranged in two halves which can be displaced longitudinally relative to each other in such a way that a winding flow area results. The flow cell has, in the measuring area proper, two measuring windows which lie opposite one another. The windows not formed with parallel planes but are spherically convex, so that they thereby function as lenses, which may have a nonnegligible influence on measurement.
In many chromatographic uses, the flow cells are made smaller still, and one speaks of micro-flow cells in this case. As a rule, a capillary tube, arranged in a housing and preferably made of glass or silica glass, as described, for instance, in "Analytical Chemistry", Vol. 56, No. 4, April 1984, pages 619A-629A, is mounted in the path of rays of a spectrometer and serves as a flow and measuring chamber. However, due to the very small dimensions of the capillary tube, problems then result with the reproducible placing thereof. Moreover, inexact gaps between the capillary tube and the detector of the spectrometer may also result. Both lead to increased noise and to drift phenomena during measurement.
A high-pressure-compatible capillary cell is required for the above-mentioned SFC-applications. It is even more unfavorable if the capillary tube is stretched within its holding device upon its placement or on connection to connecting capillary tubes;
It is an object of the invention to provide a micro-flow cell in which the fixing of a capillary tube within its holding device is simplified and is designed in such a way that a fast, reproducible connection is made possible between the holding device and the capillary tube without deformation arising, either during assembly, or due to changes in stresses during operation or by heat transfer.
It is a further object to provide a micro-flow cell which is high-pressure-compatible.
According to the present invention there is provided a micro-flow cell for chromatographic, spectrometric or scintillation measurements, comprising
a capillary tube with an inlet and outlet and defining a flow path between said inlet and outlet for a medium to be examined,
a first holding part for said capillary tube,
a second holding part for said capillary tube, the two holding parts being adapted to be secured about the length of the capillary tube to hold the latter, and being separable along their longitudinal direction,
at least one window for the transmission of measuring light to said tube formed in at least one of said holding parts,
means for connecting each end of the capillary tube in a flow path for said medium, and
means for locking said holding parts together about said capillary tube.